Field of the Invention and Related Art Statement
This invention relates to a thread joint for use in the portions of an accumulator or a cylinder in which intense load changes occur and thereby causing repeated tensions to be applied to the thread joint, and, more particularly relates to a stepped thread joint and a cutoff thread joint exhibiting a long fatigue life.
For example, an accumulator, which acts as a fluid device, is constituted in such a manner that the inside portion of the main container body thereof is divided into a gas chamber and a fluid chamber by using a bladder therein and each of the two ends of the main container body is sealed by a side plate, whereby the accumulator performs a pulsation absorbing action or a shock absorbing action by way of expanding or contracting the bladder in accordance with the fluid pressure change in the fluid circuit. In such accumulators, a parallel thread is employed so as to act as a means for securing the main container body with the side plate.
When the internal pressure of the accumulator rises and the side plate is pressed outwards, the thread is repeatedly subjected to the axial and circumferential load, that is, a so-called fluctuating load from 0 to the maximum level. These loads are not uniformly borne by the ridges of the thread, but are set off in the direction of the tensile force.
Therefore, the root of the front end portion of a female thread to which a great tension is applied generates a stress concentration, which may cause this root to be broken.
In order to overcome the above-described problem, a thread joint can be employed which is "a thread joint utilizing male thread in the shape of the tapered tip thereof that can exhibit fatigue resistance" disclosed previously in U.S. Pat. No. 4189975 and Japanese Patent Publication No. 56-53651.
A group including the inventor of the present invention, as shown in FIG. 11, formed a female thread 2 of a main container body 1 and a male thread 4 of a side plate 3 each of which was in the form of a triangular thread M106.8 X 2. Then, a test accumulator having the male thread 4 which was so constituted that the height h of ridges m.sub.7 to m.sub.1 was gradually decreased in accordance with the description in the above-described patent and a conventional accumulator in which the above-described triangular thread was employed were manufactured. The load-bearing ratio of each of the threads of the accumulators and the fatigue life were then examined under conditions of: seal diameter d=104 mm; internal pressure p=0 to 318 kg/cm; and frequency 2.5 Hz.
As a result, although the test accumulator displayed rather more uniform load-bearing ratio than the conventional accumulator, the test accumulator displayed only a shorter fatigue life than the conventional accumulator.
The thread which bore the largest load-bearing ratio was the second ridge m.sub.2 from the frontmost portion 2m in the test accumulator, the rate being 18.5%, while, in the conventional accumulator, it was the frontmost ridge, the rate being 21%. The fatigue life was, in the conventional accumulator, 560,000 times, while it was 380,000 times in the test accumulator.
When the load-bearing ratio of the thread is decreased, the fatigue life of the thread is usually lengthened. However, the fatigue life of the above-described test accumulator was shortened.
As a result of determining the cause, the peak bending moment of the maximum bending moments to be applied to the roots f.sub.1 to f.sub.10 respectively was applied to the root f.sub.2 of the second female thread from the frontmost portion 2e. The bending moment amplitude also becomes maximum, causing this portion to be broken. That is, when the male thread 4 is pressed in direction Y, each ridge fm of the female thread is brought into a cantilever state in which the lower surface thereof bears the distributed load. The height fh of the ridge fm of the female thread can be assumed to be the span which affects the size of the bending moment.
Therefore, in a case where the ridge height fh is made uniform, if the load to be borne by each ridge is not uniform, then the greater the load to be borne, the greater the peak bending moment. Furthermore, the bending moment amplitude reaches a maximum, and the thread can be easily broken. In other words, there is a maximum bending moment generated at each root of the female threads. The maximum bending moment per unit area is the maximum bending moment generated at a root of a female thread divided by the contact length of each female thread.
Each of the maximum bending moments per unit area generated at the root of the female thread of the conventional accumulator and the test accumulator was obtained by using the load-bearing ratio and the mean contact height. The result is shown in FIG. 4. In this figure, symbol A represents the maximum bending moment of the conventional accumulator, while symbol B represents that of the test accumulator. A peak bending moment PB of 13.5 kg mm/mm was generated at the root f.sub.2 of the second female thread from the frontmost thread 2e of the test accumulator B. The thus-generated moment PB was larger than the peak bending moment PA=11.4 kg mm/mm of the conventional accumulator. Therefore, the moment amplitude was also great, causing the thread to be easily broken.